Various pressure swing adsorption processes are known for separating gas mixtures. A subset of the generic pressure swing adsorption process is vacuum swing adsorption, wherein the pressure swing or pressure variation over the entire process cycle sequence includes at least some operation below ambient pressures, thus the vacuum terminology. Vacuum swing adsorption may include elevated pressures above ambient pressures in some portions of the overall cycle sequence, but must include at least some sub-ambient pressures at some portion of the cycle sequence in contrast to most pressure swing adsorption cycles, which have a lowest most pressure at ambient or above ambient pressure conditions.
In the use of pressure swing and vacuum swing adsorption techniques for the production of purified gas compositions from feed gas mixtures of bulk compositions, very high purity gas composition products are often difficult to obtain due to the tendency for co-adsorption of the components of the gas mixture and the modest selectivity of chosen adsorbent materials for one gas component over the other gas components in the total feed gas mixture. To enhance purities to commercially desirable levels, it has previously been typically believed that a purge or rinse gas step with the more selectively adsorbed component of the feed mixture in the overall pressure swing or vacuum swing adsorption cycle sequence is necessary to flush co-adsorbed components and void space gas (that gas left in the interstices between adsorbent particles and that gas in the macropores within the particles in the packed adsorption bed) prior to removing the desired gas product when the desired gas product is the more selectively adsorbed rather than the less selectively adsorbed species.
Such vacuum swing adsorption cycle sequences are capable of providing gas products of purities exceeding 99.5%. However, the use of rinse gas or purge gas particularly when it is a portion of the desired gas product results in recycle of the rinse gas to feed which in turn effects energy requirements of the overall process for the recycle, as well as enlarging the capital requirements of the process.
In U.S. Pat. No. 4,077,779 a process is described wherein hydrogen-containing gas mixtures are subjected to selective adsorption in a pressure swing cyclic system to remove carbon dioxide and/or hydrocarbon gases, obtaining high recovery of hydrogen at high purity. The system can also be employed for separation of methane from admixture with CO.sub.2. Multiple stages of depressurization are set forth but are preceded by rinse steps. The product is not selectively adsorbed by the adsorption bed, and most of it passes through the bed during the adsorption step.
Published European application 0 193 716 has a process described for separating gas mixtures containing a primary gaseous component and a secondary gaseous component by selective adsorption of the secondary gaseous component in an adsorptive process including the steps of adsorption in at least one adsorbent bed, rinsing said bed with secondary component, depressurizing and evacuating said bed without additional rinsing and repressurizing said bed with primary gaseous component. The process is particularly attractive for recovering carbon dioxide and methane from landfill gas.
Exemplary prior art systems for recovery of high purity nitrogen using vacuum swing adsorption with recycled rinse gas is described in U.S. Pat. No. 4,013,429 and the improvement patent which provides a dried nitrogen product set forth in U.S. Pat. No. 4,264,340.
Both of these patents operate vacuum swing adsorption processes with two parallel trains of two beds in series. The beds in series include a first water and carbon dioxide selective bed where feed is first introduced and second a nitrogen selective bed where nitrogen is selectively adsorbed from oxygen. After the adsorption stage of the process, a portion of the product nitrogen previously obtained is passed through the beds to rinse out co-adsorbed oxygen and void space gas from the beds prior to the desorption and recovery of the nitrogen product. The rinse gas effluent from the rinsed beds may be recycled as feed gas. This rinse requirement requires additional manifolding, valving and storage vessels and in some instances may require additional compressor or blower equipment. The rinse stage additionally complicates the cycle.
U.S. Pat. No. 4,770,676 discloses a staged series of beds for CO.sub.2 and CH.sub.4 separation in which the first in the series of beds operate without a rinse or purge step. The beds are regenerated by depressurizing to ambient to remove one portion of the CO.sub.2 and then evacuating to remove a second portion at the CO.sub.2.
Currently, pressure swing and vacuum swing adsorption techniques for production of nitrogen in relatively low volumes are experiencing strong competition for reduction in complexity, reduction in capital cost and reduction in power requirements. A premium is placed on the least complex systems with the lowest initial purchase cost and the lowest operational power requirements to deliver modest quantities of nitrogen gas. A need has arisen for a pressure swing or a vacuum swing adsorption process that will meet these competitive pressures and still provide relatively high purity nitrogen, short of ultra high purity nitrogen requirements which are limited to specialized industries such as the electronics industry and which industries are best serviced by nitrogen provided from large scale cryogenic distillation operations.
The present invention meets the needs of this identified sector of the nitrogen consuming commercial marketplace with a unique combination of vacuum swing adsorption techniques for the recovery of relatively high purity nitrogen gas from air, as will be set forth below in greater detail.